1. Field of the Invention
The present invention relates to an ultrasonic probe apparatus and an ultrasonic diagnostic apparatus which perform ultrasonic observation and diagnosis using a capacitive micromachined ultrasonic transducer (hereinafter abbreviated as “c-MUT”) which is pivotably supported at an end of an insertion section inserted into the body cavity.
2. Description of the Related Art
Prime importance is being placed on an ultrasonic diagnostic technology whereby for example, forceps of an endoscope are inserted, an ultrasonic transducer mounting part is caused to stick out at an end thereof, make contact with body tissues, for example, a stomach wall and thereby visualize information on the depth of the stomach wall, for example, a layered structure of the mucous membrane by means of ultrasound waves with high resolution.
On the other hand, a harmonic imaging diagnosis using a harmonic signal is becoming a standard diagnostic modality in recent years because it can obtain a clear diagnostic image which could not be obtained by a conventional B mode diagnosis.
The harmonic imaging diagnosis can be classified under two categories; (1) a tissue harmonic imaging method of separating, when an ultrasound wave is propagating in a body, harmonics superimposing on a fundamental ultrasound wave under the nonlinear influence of the body tissue using various methods and constructing an image using this signal and (2) a contrast harmonic imaging method of injecting contrast medium bubbles into the body, receiving harmonics produced at the moment bubbles burst or resonate through irradiation of transmission ultrasound waves, separating harmonics superimposed on fundamental ultrasound waves using various methods and constructing an image using this signal.
All these methods are proven to have such a high S/N that cannot be obtained from a conventional B mode tomogram and be able to obtain a diagnostic image with high resolution, thus contributing to improvement of the accuracy of medical diagnostics.
As for an ultrasonic transducer which is used for a conventional extracorporeal harmonic imaging diagnostic apparatus, for example, an identical transmission/reception dual-function ultrasonic transducer has been used for both transmission of fundamental waves and reception of harmonics. It is also possible to adopt a structure whereby an echo of an ultrasonic pulse reflected from a body tissue is received through an ultrasonic transducer provided apart from the one for transmission.
Since the signal level of a harmonic signal is much smaller than that of a fundamental wave, it is necessary to efficiently remove a fundamental wave component involved in deterioration of a harmonic image. For this purpose, a publicly known harmonic component (especially second-order harmonic component) extraction technology is used.
As an ultrasonic transducer, a c-MUT obtained by forming on a silicon semiconductor substrate using silicon micromachine technology is becoming a focus of attention in addition to a conventional piezoelectric type ultrasonic transducer.
As examples of prior arts of the c-MUT, National Publication of International Patent Application No. 2004-503312 and National Publication of International Patent Application No. 2004-503313 disclose the following. That is, each of a plurality of c-MUTs has a charged diaphragm and this charged diaphragm faces an oppositely charged substrate so as to produce a capacitance in between. This diaphragm is attracted to the substrate with bias charge. This substrate has a central part which rises toward the center of the diaphragm so that the charge of a cell has a maximum density in the center of the movement of the diaphragm. For a harmonic operation, a drive pulse applied to the cell is distorted beforehand in view of a nonlinear operation of the apparatus to reduce mixing of a distortion component of a transmission signal into a harmonic frequency band. A transmission ultrasound wave into which this distortion component has been mixed contains a high frequency component from the beginning, and therefore it is impossible to distinguish whether the received high frequency component detected is the high frequency component mixed from the beginning or the high frequency component is originated from the body. Since the c-MUT cell can be worked on through a conventional semiconductor process, it can be united with an auxiliary transducer circuit such as a bias charge regulator. The c-MUT cell can be further processed through micro stereo lithography, and therefore the cell can be formed using a variety of polymers and other substances.
The c-MUT generally requires not only a RF pulse signal but also a DC bias voltage to generate an ultrasound wave during both transmission and reception. That is, the c-MUT generates a signal composed of a DC bias voltage superimposed on a RF pulse signal, applies the signal to the c-MUT and thereby transmits/receives ultrasound waves.
Using a harmonic imaging technology requires an ultrasonic transducer having a wideband characteristic, and since the c-MUT has a wideband characteristic, it is suitable for a harmonic imaging diagnosis.